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Thermosonic flip-chip (TSFC) bonding is a developing microelectronic packaging technology. To provide clear understanding of bonding process and bonding mechanism, the ultrasonic vibration at bonding interface was studied. The TSFC bonding was performed on a lab bonder, the ultrasonic vibration of tool and chip were measured by using a laser doppler vibrometer, and the effect of bonding force on chip vibration velocity harmonics was analyzed. Experimental results show that the bonding strength forming process can be divided into four stages. The abrupt “amplitude dropping” of chip vibration was observed, and could be considered as a sign of bonding strength formed. The bonding strength is formed after the bonding starts 6-8 ms. This moment can be indicated by a peak of fundamental and a ramp-up of third harmonics of chip vibration velocity. The results also show that relatively small bonding force was good for forming initial bonding strength. Then the gradually increased bonding force loading is thought to be more suitable for better bonding strength and reliability.